In 1958, Ingpen was appointed by the Commonwealth Scientific and Industrial Research Organisation (CSIRO) as an artist to interpret and communicate the results of scientific research. From 1968 Ingpen worked as a freelance designer, illustrator and author. He was also a member of a United Nations team in Mexico and Peru until 1975, where he designed pamphlets on fisheries and was involved in "a number of Australian conservation and environmental projects". He left the CSIRO to work full time as a freelance writer in 1968. Ingpen's interest in conservation issues continued, and he was one of the founding members of the Australian Conservation Foundation.
Ingpen has written and/or-illustrated more than 100 published books. These include children's picture books and fictional stories for all ages. His nonfiction books mostly relate to history, conservation, environment and health issues.
Ingpen has designed many postage stamps for Australia. Ingpen designed the flag and coat of arms for the Northern Territory. Ingpen has created a number of public murals in Geelong, Melbourne, Canberra and the Gold Coast in Queensland. He also has designed bronze statues, which include the 'Poppykettle Fountain' in Geelong's Steam Packet Gardens (currently dry due to drought restrictions) and the bronze doors to the Melbourne Cricket Club. His most recent work is the design and working drawings for a tapestry, which was woven by The Victorian Tapestry Workshop, to celebrate the 150 years of the Melbourne Cricket Ground.
In 1982 Ingpen designed the Dromkeen Medal for the Governors of the Courtney Oldmeadow Children’s Literature Foundation. This medal is awarded annually to Australians in recognition of contributions to children's literature. He was awarded the medal in 1989 for his own work in the field.
Ingpen was illustrator for the centenary editions of J.M. Barrie's Peter Pan and Wendy and Kenneth Grahame's The Wind in the Willows for which he bases characterisations on contemporary figures and personalities.
Ingpen has also recently illustrated a picture book by Liz Lofthouse called Ziba came on a boat, this book has been nominated for many Australian awards including the Australian Children's Book Council Awards.
In 1999 Ingpen had a major retrospective exhibition in Taipei, which travelled to other regions of Taiwan for a two-month period. Also in 1999 an exhibition of the original artworks and book launch for Around the World in 80 Days was held in London.
In 2002 Ingpen had a solo exhibition in Bologna, Italy, and works from Shakespeare were exhibited in New York.
In 2002 Ingpen's work featured in the inaugural exhibition at The Eric Carle Museum of Picture Book Art, Massachusetts, with the artwork from Charise Neugebauer's - Halloween Circus at the Graveyard Lawn, being donated to the Museum by Ingpen.